As new generations of handsets and other wireless communication devices become smaller and embedded with increased applications, new antenna designs, system configurations and controlling algorithms are required to enable new capabilities and to improve QOS.
Antenna diversity schemes are used to improve the quality and reliability of a wireless communication link. In many instances, the line of sight between a transmitter and a receiver becomes blocked or shadowed with obstacles such as walls and other objects. Each signal bounce may introduce phase shifts, time delays, attenuations and distortions, which ultimately interfere at the receiving antenna. Destructive interference in the wireless link is problematic and results in degradation of device performance. A signal quality metric is often used to assess the quality of signals. Examples of such quality metrics include signal-to-noise ratio (SNR), signal to interference-plus-noise ratio (SINR), receive signal strength indicator (RSSI), bit error rate (BER) and other metrics, which are called channel quality indicators (CQI). An antenna diversity scheme can mitigate interference from multipath environments by monitoring one or more CQIs. Antenna diversity can be implemented generally in several forms, including spatial diversity, pattern diversity and polarization diversity, for example.
Spatial diversity for reception includes multiple antennas having similar characteristics, which are physically spaced apart from one another. In multipath propagation conditions, as encountered with a blocked or shadowed line of sight path, each of the multiple receive antennas experiences a different fading characteristic. Accordingly, where a first antenna experiences a significant reduction in signal reception, the second antenna is likely to receive an effective signal. Collectively, the spatial diversity scheme can provide a robust link. Spatial diversity for transmission is also effective, although link improvements may be needed in certain cases for the receive side of the base station.
Pattern diversity generally includes two or more co-located antennas with distinct radiation patterns. This technique utilizes antennas that generate directive beams and are usually separated by a short distance. Collectively, these co-located antennas are capable of discriminating a large portion of angle space and may additionally provide relatively higher gain compared to an omnidirectional antenna.
Polarization diversity generally includes paired antennas with orthogonal polarizations. Reflected signals can undergo polarization changes depending on the medium through which they are traveling. By pairing two complimentary polarizations, this scheme can immunize a system from polarization mismatches that would otherwise cause signal fade.
Each of the above diversity schemes requires one or more processing techniques to effectuate antenna diversity, such as switching, selecting and combining. Switching is one of the simple and efficient processing techniques and generally includes receiving a signal from a first antenna until the signal level fades below a threshold level, at which point active components such as switches engages the second antenna for communication with the receiver. Selecting is a processing technique that determines an optimal signal for utilization by the receiver during each predetermined time interval. Both selecting and switching techniques may utilize active components, such as switches, to select the optimal signal based on one or more CQIs. The selecting and switching techniques may be collectively called a switching technique wherein the selection of the signal for utilization is carried out by controlling the switches or other active components coupled to the antennas.
Combining is a processing technique wherein the multiple signals are weighted and combined into a signal for communication with the receiver. The weights are adjusted such that a main lobe of a receiving beam pattern is toward the desired direction and a null of the receiving beam pattern is toward the interference direction. Examples of combining techniques include a minimum mean squared error (MMSE) combining technique, a maximum ratio combining (MRC) technique and an equal gain combining (EGC) technique. An exemplary algorithm to carry out each of these combining techniques may be briefly summarized as follows. In the MMSE technique, the signals in paths are weighted where the weights are chosen to provide a minimum mean square error between the combined voltage stream and the signal. In the MRC technique, the signals in paths are weighted where the weights are chosen to be proportional to the respective signal amplitudes to maximize the output SNR. The weighted signals are then multiplied by respective phase factors prior to summing so that the signals are added in phase to maximize the gain. The EGC technique is a simplified version of the MRC technique, wherein the signals are weighted with the same factor and then multiplied by the phase factors.
Although the above techniques have been described for reception, their analogs are possible for transmit functions. Receive (Rx) diversity refers to configurations where a diversity scheme is applied for signal reception; and transmit (Tx) diversity refers to configurations where a diversity scheme is applied for signal transmission